<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="vue.js" type="text/javascript"></script>
    <script src="vue-resource.js" type="text/javascript"></script>
</head>
<body>

<div id="box">
    <input type="text" v-model="a">
    <br/>
    <input type="text" v-model="b">
</div>

<script>
    var vm = new Vue({
        el:'#box',
        data:{
            a:1,
        },
        computed:{
            b:{
                get:function(){
                    return  parseInt(this.a)+parseInt(1);
                },
                set:function(val){
                   this.a = val;
                }
            }
        }
    });
</script>
</body>
</html>